In order to deeply explore the internal law of water flow structures under the condition of vegetation lodging, experimental research on the influence of vegetation stem diameter on the characteristics of overland flow was carried out. Indoor open channel flow simulation tests were performed with a fixed slope of 1. 0%, a lodging angle of 20°, stem diameters of 2mm, 3mm and 4mm. The results show that the resistance coefficient of Darcy-Weisbach increases gradually with the water depth under the same diameter, the Reynolds number increases linearly, while the Froude number decreases as a power function. Under the same water depth, with the increase of the stem diameter, Darcy-Weisbach resistance coefficient increases, Reynolds number and Froude number decreases respectively. Moreover, if the stem diameter is increased for every 1mm, Darcy-Weisbach resistance coefficient increases 33. 76%, Reynolds number and Froude number decreases 2. 57% and 8. 33% respectively in average.